This invention relates to a block copolymer dispersion stabilizer for use in the aqueous dispersion polymerization of an ethylenically unsaturated monomer and to a process wherein an ethylenically unsaturated monomer is polymerized in aqueous media in the presence of said block copolymer dispersion stabilizer.
Many polymerization processes are used in the formation of synthetic fibers. Most step-growth polymerization reactions are carried out in homogeneous systems by simple combination of two or more monomers in the melt, in the absence of solvent, i.e., bulk polymerization. The principal advantage of a bulk polymerization is that the product obtained is essentially free of diluent or impurities and can be used directly. A disadvantage of a bulk polymerization reaction is that it is difficult to control the reaction temperature.
A wide variety of polymerization methods are used in the preparation of chain-growth polymers. The radical chain-growth polymerization of olefin monomers, for example, can be conducted in a number of different types of reaction systems, including bulk polymerization, solution polymerization, suspension polymerization, emulsion polymerization, and non-aqueous dispersion polymerization. Each of these systems has certain advantages and disadvantages.
Bulk polymerization is the direct conversion of liquid monomer to polymer in a reaction system in which the polymer remains soluble in its own monomer. As noted above, bulk polymerization systems are free from diluents but the reaction temperature is difficult to control.
In solution polymerization, a solvent is used which is capable of dissolving the monomer, the polymer, and the polymerization initiator. Diluting the monomer with a solvent causes a direct reduction in the rate of polymerization and in the viscosity of the product mixture at a given degree of conversion. These characteristics plus the ability of the solvent to act as an inert heat-transfer medium substantially solves the heat-dissipation problem encountered in bulk polymerization. A disadvantage of solution polymerization, however, is that the solvent, unless carefully selected, may show some reactivity with the active species of the polymerization reaction and undergo chain-transfer reactions. A further disadvantage, not present in bulk polymerization, is that the solvent must be removed at the end of the polymerization to isolate the solid polymer. If the solvent is an organic solvent, care must be exercised in its removal in order to avoid polluting the biosphere with undesirable contaminants. Also, the use of organic solvents adds to the cost of the process.
In suspension polymerization, the initiator is dissolved in the monomer, the monomer is dispersed in water, and a dispersing agent is incorporated to stabilize the suspension formed. All suspension polymerization processes use some type of suspending or surface-active material to keep the monomer globules dispersed throughout the reaction in order to avoid coalescence and agglomeration of the polymer. Not only does the suspension stabilizer affect the particle size and shape, but also the clarity, transparency, and film-forming properties of the resultant polymer. A variety of dispersing agents including water-insoluble, finely divided, inorganic materials, and organic materials, depending upon the monomer to be polymerized have been used as dispersing agents. Thus, for example, talc, barium, calcium and magnesium carbonates, silicates, phosphates and sulfates, as well as poly(vinyl alcohol), tragacanth gum, salts of styrene-maleic anhydride copolymers, vinyl acetate-maleic anhydride copolymers and their salts, starch, gelatin, pectin, alginates, methyl cellulose, carboxy-methylcellulose, bentonite, limestone and alumina have been used as suspending agents. A major advantage of suspension polymerization is that the polymeric products are obtained in the form of small beads which are easily filtered, washed, and dried. For reasons of cost and nonreactivity, water is a much more desirable diluent and heat-transfer medium than organic solvents. There is no problem in polluting the biosphere when removing water from the product. In suspension polymerization, there is a possibility that the suspension stabilizer system used may have impurities which retard or inhibit the polymerization. The suspension stabilizer must be carefully chosen so as not to adversely affect the resin properties such as clarity, heat stability, and electrical behavior. Some suspension stabilizers may result in a foam problem due to their action in lowering the surface tension of the aqueous phase. A defoamer can be utilized to avoid the problem of foaming, but it too may adversely affect the properties of the resultant polymer as well as add to the cost. Thus, suspension polymerization has a number of disadvantages in its commercial application.
An emulsion polymerization process is considered to be a three-phase reaction system consisting of large droplets of the monomer, the aqueous water phase containing the dissolved initiator, and the colloidal particles of monomer-swollen polymer. The monomer droplets and the swollen monomer-polymer particles are stabilized in the aqueous medium by absorbed surface-active agents incorporated into the reaction mixture before the start of the polymerization. The initiators are usually water soluble. Inorganic peroxides are quite often used as initiators. Hydrogen peroxide and ammonium peroxysulfate are specific examples of such inorganic peroxide compounds. Frequently a reducing agent is used in conjunction with a peroxide compound which combination is referred to as a redox initiator system. Because an inherent characteristic of this method of polymerization is the formation of unusually high molecular weight polymers, the use of a chain-transfer agent, termed a modifier, is often required to limit the molecular weight of the polymer without reducing the rate of polymerization reaction. While the emulsion polymerization process has the economical advantage of using water as the emulsion base, the emulsifier may give rise to problems in obtaining a solid polymer product free from undesirable contaminants. If a chain-transfer agent is required, some of the economical advantage of using water as the base is lost in the cost of the chain-transfer agent.
In non-aqueous dispersion polymerization, the reaction is effected by polymerizing a monomer in an organic liquid in which the resulting polymer is insoluble, using a dispersion stabilizer to stabilize the resulting particles of insoluble polymer in the organic liquid. A review of the published literature relating to non-aqueous dispersions and dispersion stabilizers appears in "Nonaqueous Dispersions as Vehicles for Polymer Coatings" by R. Dowbenko and D. P. Hart, Ind. Eng. Chem. Prod. Res. Develop., Vol. 12, No. 1, pp. 14-28, 1973. Since non-aqueous dispersion polymerization does not embody the process of the present invention, a detailed discussion of non-aqueous dispersion polymerization will not be presented here. While non-aqueous dispersion polymerization has a number of advantages particularly where the resulting polymer dispersion is to be used without removing the organic dispersing medium, the process has certain economical and ecological disadvantages when a free-flowing, dry, particulate powder is the desired product. Unless care has been exercised to remove any coalescing solvent prior to drying the polymer, there is a tendency for the polymer particles to agglomerate and become fused during the end of the drying process.
U.S. Pat. No. 3,580,880 teaches making amphipathic stabilizers and using them as dispersion stabilizers in either aqueous or non-aqueous polymerization processes. However, the dispersion stabilizers so obtained contain at least 25% by weight of components solvatable by a lipophilic liquid, such as, for example, aliphatic and aromatic hydrocarbons and at least 25% by weight of components solvatable by a hydrophilic liquid, such as, for example, water and strongly hydrogen bonded organic liquids which are completely miscible with water, e.g., ethylene glycol, methyl alcohol and ethyl alcohol.
In the aqueous dispersion process of the present invention, the hydrophilic or water-soluble block of the dispersion stabilizer comprises about 50 to about 97 percent, preferably about 80 to about 90 percent by weight of the stabilizer and the hydrophobic or water-insoluble block of the dispersion stabilizer comprises about 3 to about 50, preferably about 10 to about 20 percent by weight of the stabilizer in order to effect optimum dispersion in water of the polymer obtained in the main polymerization reaction. Also, the stabilizer disclosed in said patent appears to be macromonomers that are randomly copolymerized with "surface" attachment of the stabilizer to the main polymer. To the contrary, the block copolymers of the present invention comprise hydrophilic and hydrophobic blocks, the hydrophobic blocks having a plurality of pendent groups such as ethylenically unsaturated bonds and/or epoxy groups for attachment of the stabilizer to the main polymer by covalent bonds. Still further, the dispersion stabilizer in the patent is used in the main polymerization reaction as a solution of the stabilizer in an organic liquid. The stabilizer of the present invention is prepared in an organic liquid but is dried so that the organic liquid is removed before it is used as a dispersion in the main polymerization reaction. Thus, the present invention effects an aqueous dispersion polymerization reaction in the absence of an organic diluent capable of polluting the atmosphere if not properly contained.